For a liquid crystal display device, a backlight unit that emits light from a backside of a liquid crystal display panel to illuminate the liquid crystal display panel has been used. The backlight unit is configured by using a light guide plate for diffusing light emitted from a light source for illumination to illuminate the liquid crystal display panel, and optical members such as a prism sheet and a diffusion film for uniformizing the light emitted from the light guide plate.
The current mainstream of a backlight unit of a large-size liquid crystal television set is a so-called direct type system in which the light guide plate is not provided and an optical member such as a diffusion plate is provided just above a light source for illumination. In the system, a plurality of cold cathode tubes serving as light sources are arranged to a rear surface of the liquid crystal display panel and a white reflection surface is provided in an inner portion to ensure a uniform light amount distribution and necessary brightness.
In order to obtain the uniform light amount distribution in the direct type backlight unit, a thickness in a direction perpendicular to the liquid crystal display panel is required to be a predetermined thickness, for example, approximately 30 mm. A thinner backlight unit may be desired in the future. However, according to the direct underneath type, in view of light amount unevenness, it may be difficult to realize a backlight unit having a thinner thickness, for example, a thickness equal to or smaller than 10 mm.
Therefore, a backlight unit using a light guide plate in which scattering particles for scattering light are mixed into a transparent resin is proposed (see, for example, JP 07-36037 A, JP 08-248233 A, JP 08-271739 A, or JP 11-153963 A).
For example, JP 07-36037 A discloses a light scattering guide light source device characterized by including: a light scattering guide having at least one light incident surface region and at least one light extraction surface region; and a light source means for causing light to be incident on the light incident surface region, in which the light scattering guide has a region whose thickness is reduced with an increase in distance from the light incident surface region.
JP 08-248233 A discloses a plane light source device which includes: a light scattering guide; a prism sheet provided on a light extraction surface side of the light scattering guide; and a reflector provided on a rear surface side of the light scattering guide. JP 08-271739 A discloses a liquid crystal display provided with a light exit direction adjustment element which includes a light incident surface having prism-row-shaped repeated unevenness and a light exit surface with light diffusion and is made of a plate-shaped optical material. JP 11-153963 A discloses a light source device which includes a light scattering guide whose inner portion has scattering power and a light supply means for supplying light from an end surface portion of the light scattering guide.
In the planar illumination devices as described above, light which is emitted from the light source and enters the light scattering guide from the light incident surface undergoes a single or multiple scattering action at a constant rate while the light propagates through the inner portion thereof. A substantial part of the light which reaches both surfaces of the light scattering guide or a surface of the reflector undergoes a reflection action to return to the inner portion of the light scattering guide.
During such a complex process, a light beam emitted from the light extraction surface at high efficiency is produced with the directivity to a forward oblique direction as viewed from a direction of the light source. In other words, the light emitted from the light source is emitted from the light extraction surface of the light scattering guide.
As described above, it is disclosed that the uniform light may be emitted at high emission efficiency when the light guide plate mixed with the scattering particles is used.
A planar illumination device is disclosed in which, in addition to a light guide plate having a region whose thickness is reduced with an increase in distance from the light incident surface, a flat-shaped light guide plate, or a light guide plate fit to the light guide plate having the region whose thickness is reduced with the increase in distance from the light incident surface is used as a light guide plate.
Patent Document 1: JP 07-36037 A
Patent Document 2: JP 08-248233 A
Patent Document 3: JP 08-271739 A
Patent Document 4: JP 11-153963 A